Render times changing over course of render
Render times are different for each frame, depending on the complexity of the content.

Any change inside the scene and within the image has to be computed separately.

Estimated rendering times are never accurate for the same reason, you can have a relatively simple scene to render at the beginning of the shot, and then have an explosion with particles and all kinds of complications or motion blur, those will take longer (sometimes much, much longer) to render.

3D Rendering is quite demanding in terms of resources and will push any computer. When computers get hot they go into a slower speed in order to protect the hardware. If the hardware reaches a critical temperature it will fail and shut down. If you experience abnormal heat and the computer is "throttling"  you need to invest in some kind of cooling system.